1. Field of the Invention
The present invention relates to an optical head for recording and reproducing data from an optical recording medium and an optical data recording/reproducing apparatus using the same. More particularly, the present invention relates to an optical head capable of detecting an error in the thickness of the substrate of an optical recording medium and an optical data recording/reproducing apparatus using the same.
2. Description of the Background Art
Recording density available with an optical data recording/reproducing apparatus is inversely proportional to the square of the diameter of a beam spot formed by an optical head on a disk or optical recording medium; the smaller the beam spot diameter, the higher the recording density. The beam spot diameter is, in turn, inversely proportional to the numerical aperture of an objective lens included in the optical head; the greater the numerical aperture, the smaller the beam spot diameter.
On the other hand, if the substrate of the disk has thickness different from a designed value, the resulting spherical aberration disturbs the shape of the beam spot and thereby deteriorates data recording and reproducing characteristics. Spherical aberration is proportional to the fourth power of the numerical aperture of the objective lens, so that a margin as to the thickness error of the disk with respect to the data recording and reproducing characteristics decreases with an increase in the numerical aperture of the objective lens. Therefore, the prerequisite with an optical head and an optical data recording/reproducing apparatus including an objective lens whose numerical aperture is increased to increase recording density is that the thickness error of the substrate be detected and compensated for.
Japanese Patent Laid-Open Publication No. 2000-40237, for example, discloses an optical head constructed to detect the thickness error of a substrate included in a disk. The optical head disclosed includes an objective lens pair made up of a first and a second objective lens facing a disk. A first actuator drives the first objective lens in the direction of optical axis. A second actuator is loaded with the first actuator and second objective lens and drives them in the direction of optical axis. The first actuator varies a pitch between the first and second objective lenses while the second actuator varies a pitch between the objective lens pair and the disk. To detect the thickness error of a disk, the amplitude of an RF (Radio Frequency) signal recorded in the disk is observed. To compensate for the thickness error, the optical head adjusts the pitch between the objective lenses and in such a manner as to produce spherical aberration that cancels spherical aberration ascribable to the thickness error.
The problem with the conventional optical head described above is that an RF signal cannot be observed unless it is recorded in the disk beforehand. A read-only disk stores an RF signal beforehand and therefore allows the thickness error of its substrate to be detected. However, a thickness error cannot be detected when it comes to a write-once disk or a rewritable disk that does not store an RF signal beforehand. The above optical head may be constructed to record an RF signal in a disk and then detect the thickness error of the disk. However, such an optical head does not compensate for a thickness error and therefore disturbs the shape of a beam spot due to spherical aberration ascribable to the thickness error, failing to accurately record an RF signal.
Japanese Patent Laid-Open Publication No. 2000-57616, for example, teaches an optical head configured to detect a thickness error even with a disk not storing an RF signal. This optical head receives light reflected from a disk by dividing it into a center portion and a peripheral portion. This brings about a problem that a particular current-to-voltage converting circuit must be assigned to each of a photosensitive portion responsive to the center portion and a photosensitive portion responsive to the peripheral portion. As a result, the optical head needs two times more current-to-voltage converting circuits than a conventional optical head. Moreover, such a number of current-to-voltage converting circuits aggravate noise ascribable thereto and thereby reduce the S/N (Signal-to-Noise) ratio of the RF signal.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 9-147369, 11-195229, 2000-20993 and 2000-30290.
It is therefore an object of the present invention to provide an optical head capable of detecting a thickness error even with an optical disk not storing an RF signal, and providing an RF signal with a desirable S/N ratio.
In accordance with the present invention, an optical head includes a light source, an objective lens for condensing light output from the light source on an optical recording medium, and a photodetector to which light reflected from the optical recording medium is incident. The light output from the light source is split into a main beam and a sub-beam different from each other in light intensity distribution when input to the objective lens. A focus error signal representative of the shift of a focus on the optical recording medium is detected from each of reflected light derived from the main beam and reflected light derived from the sub-beam. An error in the thickness of the optical recording medium is detected on the basis of a difference between the zero-crossing point of the reflected light derived from the main beam and the zero-crossing point of the reflected light derived from the sub-beam.
Also, in accordance with the present invention, in an optical data recording/reproducing apparatus including a light source, an objective lens for condensing light output from the light source on an optical recording medium, and a photodetector to which light reflected from the optical recording medium is incident, the light output from the light source is split into a main beam and a sub-beam different from each other in light intensity distribution when input to the objective lens. A focus error signal representative of the shift of a focus on the optical recording medium is detected from each of reflected light derived from the main beam and reflected light derived from the sub-beam. An error in the thickness of the optical recording medium is detected and compensated for on the basis of a difference between the zero-crossing point of the reflected light derived from the main beam and the zero-crossing point of the reflected light derived from the sub-beam.